Weight and Balance made difficult

[AGB]

Hello,
My friend showed up at the airport with a set of scales. I jumped at the opportunity and decided to weight the plane, a standard 170A. Well I knew it had to be weighted with unusable fuel in it, so I did what I thought unusable meant. With all gears on ground (3 point attitude) drained all the fuel through the gascolator. The total was 1388lb, which I think is a little too high. From reading many topics on this forum, I am wondering if there was more fuel in the tanks than unusable.

- Does anyone know how much fuel is left on the tanks when one drain thought the gascolator?
- I understand now, thanks to this forum, that it would have been easer to weight with full tanks or empty, but how do you leave in just the unusable in the tanks?

Thanks and good flying,
Donovan

[GAHorn]

Weigh the airplane with FULL fuel, and then subtract the useable fuel of 37 gallons. Simple, heh?
(Hint: You now are questioning whether or not you actually had only unuseable fuel onboard, and do not have any reliable method to confirm your suspicion that your airplane is actually heavier than expected. Chances are...your airplane is heavier than you wanted to believe. Now you must accept the 1388 empty wt....which is actually not all that different than most others. My own is 1348 and I only have scant and light radios and almost no paint.)

[Bruce Fenstermacher]

Donovan what you could do if you've already added fuel since weighing is drain it again through the gasolator at the three point attitude in the same place so it's position is the same. Then add fuel till full and do the math to find what was in the tanks when it was weighed. Not perfect but should be close enough.

[KS170A]

If you're just wanting to know the weight of the airplane, weighing in the 3-point attitude will work. But to find the weight AND center of gravity, it should be placed in flight level attitude, with unusable fuel (fuel drained in the level attitude), and (for this vintage of airplane) no oil in the engine. Granted, most weighings these days probably add in the oil to make it simpler. I've attached a blank form for the 170 model.

[AGB]

Bruce,
Good idea, don’t know if will ever get around to do it though.

Thank you all for the replies!

Best Regards,
Donovan

[Bruce Fenstermacher]

I'm assuming as Josh pointed out that you leveled the aircraft in order to figure its arm.

As Josh points out using the method I suggest to find the amount of fuel you had when weighed will only allow you to figure the weight of the aircraft not the arm. But since Cessna only provided one arm, at 48 inches for all the fuel except unusable, you could do the math for tha arm and be reasonably close. I don't know it I'd use it all for an offical W&B as there are to many bandaids to the method.

[jrenwick]

<soapbox>
In my opinion (be it ever so humble, because I'm not an AMT) there's no real value to weighing an aircraft unless it's done by a licensed mechanic and results in a legal W&B sheet in the aircraft and logbooks. Weighing an aircraft without doing it to the letter and documenting it in the logs is like George's take on a "pre-buy" vs. annual inspection. One thing has real value, the other, not so much.

In my career as a software engineer I formulated "Renwick's Law of Software" (never published) which states that software always becomes larger and slower over time unless something is done specifically to make it smaller or faster. Aircraft obey a similar law, in that they always become heavier over time unless an effort is made to lighten them. Maybe the same rule applies to speed, but I haven't observed that. Speed seems to be more dependent on the color of the paint, or lack of it (from anecdotal evidence).

There are only a few maintenance events that absolutely require weighing; some of them are restoration, painting, recovering of fabric aircraft, or washing a rat plane with WD-40. In any other case, the new W&B should be calculated. To do otherwise is to invite disappointment, as George points out above, or doubt due to the uncertainty of the weighing being done under the same conditions as the official W&B -- you'll just never know, and the weight you got unofficially need not affect how you load the airplane, anyway.

If you have some reason to doubt the official W&B in your aircraft, you could put it on scales to double-check -- but if the difference is more than a few pounds, you'll need to have a mechanic do it right and update the aircraft's documentation before it means anything at all.
</soapbox>

[Bruce Fenstermacher]

I remember having this discussion before. I seem to remember asking for evidence that an A&P license was required to weigh an aircraft and create a weight and balance. I recall George presented something that he concluded showed an A&P license was required. George do you remember what that evidence was that I didn't quite buy into. I seem to remember the issue was unresolved, perhaps just in my eyes.

[GAHorn]

I don't recall the conversation, Bruce.
But the only "official" reason an aircraft changes weight or balance is due to the addition or removal of equipment, or major alterations/repairs. Therefore I think it reasonable that only persons authorized to perform those actions would qualify to record the data.
Personally, if I found previous data to be in error, as the owner/operator I believe I'd have the responsibility to correct it. Then at the next annual inspection I'd have the aircraft re-weighed and have the inspector enter the data.

[Bruce Fenstermacher]

Yes George I believe that was your take on it last time, that in most cases other maintenance was being performed that required a new W&B to be calculated or the aircraft weighed and it is this other maintenance that required the license for the whole operation. This wouldn't address the case were no maintenance was performed but the owner/pilot wanted to reaffirm the data.

I think it an interesting gray area. A pilot is trained to calculate a W&B and expected to perform this function as part of his PIC responsibilities. Reading FAA-H-8083-1 Aircraft Weight and Balance handbook says in a paragraph that it is the responsibility of the AMT (Aviation Maintenance Technician) to provide the empty weight and CG of the aircraft to the pilot and in another spot it says the same thing but listing it as a responsibility of the AMT after repair or maintenance that would cause a change. The handbook does not address under what FAR i.e 91, 135, 121 the aircraft is flown when making this statement. Certainly the relationship of the owner/pilot and the aircraft is different under each of these regulations.

Under FAR 43 just weighing an aircraft is not a major alteration nor is it listed as preventive maintenance. I wonder if weighing and calculating the W&B is maintenance at all? Interesting though that item 18 under preventive maintenance allows the pilot/owner to switch between wheels and skis when no W&B computation is involved. Even then it doesn't say the pilot can't ever do the computation, it is just the limit set for that maintenance involved.

Under FAR 43 preventive maintenance there are a few things that a pilot/owner is specifically allowed to do with no limitations as far as changing the W&B. Changing tires with heavier recaps could happen and require the W&B to be recomputed. Changing out the entire interior under the "repair" allowed by FAR 43 appendix A I would think would certainly require a new W&B.

So while an A&P license is traditionally thought of as being required to perform a W&B I'm not so sure that under all circumstances that is the case.

[jrenwick]

I don't think this is really a gray area. First, let's be sure to distinguish clearly between a) the Weight & Balance Report, along with the list of equipment included in the empty weight, that is required to be in the airplane at all times, and b) the calculation of the loaded weight and C.G. for any given flight. B) is a pilot action, obviously -- so let's first agree that we're talking about the preparation of a), the required empty weight and balance report.

This requirement is from NOTE 1 of the TCDS for our aircraft (A-799), which reads, in part: "Current weight and balance report including list of equipment included in certificated empty weight, and loading instructions when necessary, must be in each aircraft at the time of original certification and at all times thereafter (except in the case of air carrier operators having an approved weight control system)." The a) document is part of what makes the airplane airworthy, and for that reason, I'd bet the FAA regards its preparation as a maintenance action. It isn't included in the list of preventive maintenance actions in Part 43 Appendix A (c), so it isn't something an owner/pilot can do. It requires an AMT to prepare it, and without the AMT's signature, date and certificate number, the airplane isn't airworthy.

FAR Part 23 also addresses the requirement for weight & balance data. I found a very helpful writeup on this here: http://forums.jetcareers.com/general-to ... ments.html

[GAHorn]

Bruce, I believe Preventive Maintenance refers to replacement of identical equipment, for example refurbishment of aircraft seats would be renewing them to their previous condition...not replacing them with heavier/lighter or different model seats. Same thing with tires. As much as I anticipate a huge uproar over this, I believe the pilot may replace tires of same size, etc.. under preventive maintenance. Changing to a different tire size is a modification and not preventive maintenance, regardless of whether the basis of approval is the TCDS or an STC, etc..

[Brad Brady]

Bruce, I believe Preventive Maintenance refers to replacement of identical equipment, for example refurbishment of aircraft seats would be renewing them to their previous condition...not replacing them with heavier/lighter or different model seats. Same thing with tires. As much as I anticipate a huge uproar over this, I believe the pilot may replace tires of same size, etc.. under preventive maintenance. Changing to a different tire size is a modification and not preventive maintenance, regardless of whether the basis of approval is the TCDS or an STC, etc..

George,
I don't think .....if I'm reading this post right, there should be any uproar on Preventive Maintenance, Your right on your take as far as I know. Every now and again a weight and balance should be done on an aircraft. ( I Have seen many pounds difference the actual, and the paper work, on several aircraft I have weighted, going both ways!!!) Remember that the scales don't have to be certified for part 91, But the person certifying the weights has to be.....(IE, AP or IA) After all these are documents that relate to that certain aircraft, and have to be singed off by a certified person......Brad

[blueldr]

I've seen a number of airplanes come out of a weighing a little too much on the heavy side to suit the needed pay load. Subsequently an adjustment of the empty weight was accomplished by pencil to accomodatethe required loads. Most airplanes, and their pilots, seem to gain weight with age.

[Bruce Fenstermacher]

Well George it should be no surprise that I don't agree. No where in appendix a does it say the preventive maintenance or repair has to weigh the same in order to qualify. Can I change tires with like size tires but recapped and heavier?